The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
(1) Field of the Invention
The present invention relates to an optical sensor system and more particularly to an electrically passive, remote hydrophone arrangement which produces stable acoustic signals using only inboard electronic signal processing.
(2) Description of the Prior Art
Early optical fiber type arrays using all-optical hydrophones and optical leads such as U.S. Pat. No. 4,115,753 did not provide for suppression of the phase and intensity noise introduced into the outboard leads. More recently, electrically passive all-optical hydrophones have been described in co-pending patent application Ser. No. 274,034. These hydrophones utilize laser frequency tuning to facilitate feedback stabilization of discrete two-beam interferometric sensors. Further, co-pending patent application Ser. No. 537,752 described parallel operation of several such sensors energized by one stable light source utilizing laser frequency modulation by a periodic signal in conjunction with a signal demodulation technique that removes low frequency phase noise from the signal. This concept also appears in, IEEE Journal of Quantum Electronics, QE-18, pp. 1639-1644, 1982. An alternative technique for demodulation was described by A. Dandridge, et al., IEEE Journal of Quantum Electronics, QE-18, 1647-1653, 1982. The Dandridge et al technique of phase modulation within a two beam interferometer by means of a piezoelectric device is inconsistent however with electrically passive outboard operation of that sensor. Such piezoelectric phase modulation allows matching of the lengths of the two paths of the interferometer so that the interferometer then becomes insensitive to the phase noise of the laser in the frequency band of the acoustic signal. Lagakos et al, in IEEE Journal of Quantum Electronics, QE-18, 1633-1638, 1982, describes extended microbend, intensity type hydrophones operated by transmission of light through multimode fibers. These sensors, while electrically passive, require a pair of parallel leads for each hydrophone. Although multimode fiber leads are believed to be robust versus mechanically excited noise as compared to single mode fibers, they do not provide inherent immunity to such noise in the sensor technique taught by Lagakos et al.
Accordingly, it is a general purpose and object of the present invention to provide an electrically passive, remote, single outboard lead optical hydrophone array, wherein, the elements of the array are arranged in a serial and/or a parallel configuration. It is a further object to show that stable acoustic signals may be derived from interferometric optical hydrophones, i.e., phase sensors, without the need for matching outboard acoustically isolated reference paths. A still further object is that the invention derives stable acoustic signals by totally inboard optical and electronic processing from an array of phase sensors and from an array of intensity (microbend) sensors.
These objects ate accomplished with the present invention by providing an optical waveguide hydrophone array, accessed remotely by a single mode fibers which transmits a light pulse sequence to the array and receives sound modulated return light signals. Within each phase hydrophone array, butt coupled fibers form a continuous structure, encapsulated within a compressible plastic tube. Tube volume expands and contracts linearly with variations in acoustic pressure, thereby proportionally modulating the optical path therethrough. Each joint, that couples hydrophones reflects part of the incident light beam back via the fiber lead to an interferometer, which compensates for path differences between equally spaced array joints. The recombined beam passes through a light detectors and a signal processor. Interference is produced between reflected light beams from successive joint pairs, from which phase modulation due to each hydrophone may be obtained. The partially reflecting joints thus permit sampling of the phase modulation of light as a function of position. Light reflected from the fiber lead/array joint provides a signal that is proportional to light source intensity variations and perturbation effects experienced by the lead. This intensity noise therefore may be cancelled. Since the fiber lead provides a common conductor for phase modulated light beams from all joints, lead induced phase noise is not present in the signal. Also, since path differences between interferring beams are matched precisely by a compensating interferometer, light source phase noise is cancelled.